The present invention relates generally to drive mechanisms for steering columns and more specifically to systems and methods for adjusting the position of a worm drive gear of a power assist steering system relative to a worm gear driven by the worm drive gear.
In many vehicles, a steering system includes a steering shaft (e.g., a steering wheel supported by a steering column and coupled to an intermediate steering shaft) whose rotation is linked to one or more steerable vehicle wheels. As the steering shaft is rotated, such as by an input from a vehicle operator or an automatic steering controller, the orientation of the one or more steerable vehicle wheels is changed so as to cause the direction of the vehicle to change. For example, vehicles commonly include a rack that is coupled to a pair of the vehicle's front wheels, with the rack being caused to move from side to side through the action of the intermediate steering shaft, which is caused to rotate about its central axis with rotation of the steering wheel.
Today's steering systems often include a power assist system to aid in rotating the intermediate steering shaft and therefore in moving the vehicle's steerable wheels from one orientation to another. In such systems, a torque assist system may include a hydraulic or electric drive mechanism (e.g., a torque-assist motor or pump) that applies a torque to a steering-assist drive shaft, causing the steering-assist drive shaft to rotate about its central axis. A worm may be disposed on the steering-assist drive shaft so as to facilitate application of a steering assist torque to a worm gear that is coupled to the intermediate steering shaft. The application of the torque assists in changing the orientation of the vehicle wheels. In such vehicle steering systems, which may include power assisted steering systems such as electric-assist power steering systems, the fit (i.e., meshing) between the worm and the worm gear can significantly impact the responsiveness and feel of the steering system as well as the amount of play (i.e., hysteresis) in the system. Accordingly, it is desirable to have a worm and worm gear combination with a close mesh (i.e., exhibiting small levels of clearance between gear teeth of the two components).
In production of large quantities of components, variations in critical dimensions may occur from one part to the next. As a result, without additional measures being taken to match individual components, undesirable variations in clearances between gear teeth of a particular worm and randomly chosen worm gear to be paired with that worm. To address such production variations, a number of techniques have been employed. One exemplary technique involves first assessing the dimensions (i.e., “sizing”) individual worms and worm gears and then matching components to produce combinations with desirable fit characteristics (e.g., levels of clearance between the worm and teeth of the worm gear).
In accordance with this technique, worms may be machined to exacting tolerances and then classified into one of a plurality (e.g., twelve) of size classifications. Then, gears from complementary classes may be matched to produce combinations exhibiting desirable characteristics. Also, trial and error approaches may be used wherein gears are assembled, then removed, exchanged, and re-assembled until a desired fit is achieved. Unfortunately, these techniques can be costly, labor intensive, time consuming, and unreliable.
Accordingly, it would be desirable to have an improved system and method for assembling a worm and worm gear combination to produce a power-assisted steering system having a conveniently adjustable fit.